This invention relates to turret punch presses, and more particularly to an improved method for refurbishing worn punch holder guide bores of a turret punch press and other similar machine tools.
A turret punch press operates by positioning a work piece, such as a piece of sheet metal, to be punched within a gap between generally round and horizontally spaced turrets. Each turret includes a number of bores proximate the perimeter of the turret. A bore in the upper turret has a correspondingly sized and configured bore in the lower turret. A punch holder is positioned within a bore on the upper turret and a die holder is positioned within the corresponding bore on the lower turret. When the corresponding upper and lower bores are rotated to a punch station on the punch press, a generally vertical reciprocal ram is actuated to engage a punch held by the punch holder. The ram forces the punch with the punch holder and the bore downwardly to engage the work piece. A die is secured within the die holder and the lower bore and cooperates with the punch being forced downwardly by the ram to thereby punch a specific configuration or design in the work piece. Repeated operation by the ram upon selected punches and corresponding dies in the turrets produces a punched and configured work piece as desired.
The punching operation in a turret punch press optimally requires precise axial alignment of the respective die and die holder in the lower turret, the corresponding punch and punch holder in the upper turret and, to a lesser degree, the ram. The die, die holder, punch, punch holder and ram are all preferably aligned along an axis referred to herein as the "punch axis" and the terms axial, axial alignment, axial misalignment, or derivations thereof are in reference to the punch axis throughout this document.
The repeated and forceful axial motion of the punch holder within the bore typically produces non-uniform wear on the sidewalls of the bore in the upper turret. With repeated use and over time, the upper bore is worn so that the critical axial alignment between the punch holder and the die holder is diminished and, thus, axial alignment between the punch and die is also diminished.
Specific punching operations such as "nibbling" and the rapid off-center forces delivered to the punch holder as a result of such operations tend to increase the rate of wear on the upper bore. The worn bore fails to provide a rigid guide for the punch holder and an accurate alignment of the punch with the die during the punching cycle. As a result, work piece quality is diminished and excessive tool wear or breakage is possible. Therefore, worn turret bores must be repeatedly monitored and repaired in order to maintain consistently high work piece quality.
One alternative to repairing the worn turret bores is to replace both the worn upper and mating lower turrets. This alternative is generally considered to be the most effective; however, it is extremely costly, approximately 940,000 to 965,000, and entails long lead times, approximately four to six months, for supply of the necessary parts and materials. Furthermore, replacing the upper and lower turrets results in excessive downtime, approximately two to three weeks, for installation of the turrets on the punch press.
The lower turret, although typically having bores which are less worn than the upper turret, is usually replaced along with the upper turret so that the mating bores of the lower and upper turret are compatible when they are installed on the machine. The requirement of replacing both the upper and lower turrets adds substantially more cost to the repair as well as increasing the time required for the repairs.
Another alternative for the repair of worn turret bores is to repair only the worn bores in the upper turret. Advantages associated with such repairs include avoiding the substantial disassembly of the press and resulting excessive downtime associated with replacing both the turrets. As a result, repairing only the worn bores in the upper turret provides a more durable and substantially less costly repair process than replacing the turrets.
Several alternative methods for repairing worn upper turret bores have been attempted over the years with varying degrees of success. One such method is disclosed in U.S. Pat. No. 4,240,314. This patent is directed to a centering and wear compensating means for punch presses and the like and discloses inserting plugs of resilient long-wearing, self-lubricating material into equi-angularly spaced bores of uniform length provided in the wall of a turret bore. These plugs are curved on one face to conform to the punch holder which reciprocates within the bore. Additionally, the plugs are chamfered for easy compressibility as the punch holder is positioned within the turret bore containing the plugs. The method of repairing worn turret bores according to this patent includes cutting at least three equi-angularly spaced wells of uniform depth in the sidewall of the turret bore. These wells extend into the turret from the sidewall of the turret bore in a direction substantially normal to the axis of the turret bore.
Problems associated with repairing worn turret bores according to U.S. Pat. No. 4,240,314 include the failure of the plugs inserted into the wells. The plugs tend to compress and deform under heavy loads thereby allowing the punch holder to deflect resulting in off-center punching, especially during nibbling operations. The plugs provide only a small bearing surface to support the punch holder thereby magnifying the problem of punch holder deflection under loads. Although in practice plastic plugs may improve the support of the punch holder after initial installation, the plugs quickly wear down and must be frequently replaced in due course.
Another alternative to repairing worn turret bores is disclosed in U.S. Pat. No. 4,756,630. This patent is directed to a punch holder guide which includes a plurality of preferably plastic wear strips which are secured into dove-tailed longitudinal keyways broached into the sidewall of the turret bore. The wear strips are installed within the dove-tailed keyways the entire length of the upper turret bore. The wear strips protrude inwardly from the cylindrical sidewall of the bore to create frictional contact between the strips and the punch holder during operation and thereby prevent contact between the punch holder and the sidewall of the bore.
Although the method disclosed in U.S. Pat. No. 4,756,630 provides enlarged bearing surfaces and support for the punch holder during operation, deflection of the wear strips during off-center punching produces misalignment of the punch holder, and subsequently the punch and die, because the wear strips fail to maintain a rigid support surface. The plastic wear strips are also subject to rapid wear under medium to heavy production loads of the punch press and all too soon must be replaced.
Another problem with the repair processes described in both U.S. Pat. Nos. 4,756,630 and 4,240,314 is that neither method includes a means for obtaining support for the punch holder along the centerline of the original bore. As a result, final alignment between punch and die is only randomly obtained even when the repairs are initially complete and in their most effective state prior to further use and wear.
Another method for repairing worn turret bores is disclosed in U.S. Pat. No. 5,263,237 which is directed to a method for restoring punch and die alignment for a turret punch press. The method disclosed in this patent includes a process for correcting both the angular misalignment of a turret bore relative to a reference axis and for correcting radial misalignment of a turret bore relative to a second reference axis. An initial step of the process disclosed in this patent involves the detection of any misalignment between the upper turret apertures and a point on the turret punch press described as the punch axis which is determined by the centerline of the ram aperture. This process involves removal of the ram, ram bushing, dies and die holders, and punches and punch holders. A gauge rod and dial indicator is then mounted on the turret to tram the inside diameter of either the upper turret punch guide bore or the lower turret die locating bore. A method is described for enlarging the guide bores in the upper turret using a boring bar that corrects any detected misalignment with respect to the ram aperture and the lower turret bore which have been aligned to create the reference punch axis.
The repair method according to U.S. Pat. No. 5,263,237 includes fixedly securing a cutting tool to the boring bar that is guided and centered on the punch axis by means of a fixed bushing mounted in the lower turret die holder and a bushing assembly mounted in the ram bushing aperture. The boring bar and cutting tool are coupled to a drive motor that is mounted to an extreme upper shelf of the punch press frame. The mounting of the drive motor necessitates removal of the clutch and brake assemblies, a flywheel and a crankshaft and related assemblies of the punch press at the punch station. Once the worn turret bore is re-bored with respect to the reference punch axis, an elongated cylindrical sleeve insert is positioned into the bore and extends the entire length of the bore. The inside diameter of the sleeve is sized for close fit relationship to a standard punch holder and the outer diameter of the sleeve is sized for close fitting tolerance to the enlarged turret bore in which it is inserted. The sleeve is fabricated from a durable metal which is hardened to at least the hardness of the punch holder.
Problems associated with the repair process described in U.S. Pat. No. 5,263,237 include the complicated steps of generating a reference axis which includes the upper and lower turret bores and the ram bushing aperture. Additionally, the re-boring process is accomplished at the punch station and requires major disassembly and reassembly of the punch press to complete the repairs. The major disassembly and reassembly of the punch press necessitates extensive downtime of the punch press and results in a cost ineffective repair process.
Another method is disclosed in U.S. Pat. No. 5,267,384 which is directed to a method for providing a punch holder body guide for punch presses. The method disclosed in this patent includes inserting a sleeve into a worn turret bore so that the punch holder is accurately centered and guided within the sleeved bore. The outer diameter of the sleeve is larger than the diameter of the bore, thereby requiring that the sleeve be compressed prior to insertion within the bore and then allowed to expand into an interference fit within the worn bore. Prior to compression and insertion within the bore, the sleeve is machined to the appropriate dimensions within a bore simulating holding mandrel. The sleeve may also need to be shrunk prior to insertion into the bore simulating holding mandrel. The sleeve is shrunk by immersion into a liquid nitrogen bath. Once shrunk and machined, the sleeve is inserted into an enlarged bore to accommodate the dimensions of the sleeve. After the sleeve is inserted into the enlarged bore, it is allowed to warm to ambient temperature and is elastically compressed and bound into interference fit within the bore and is virtually irremovable therefrom.
Problems associated with the method disclosed in U.S. Pat. No. 5,267,384 include the failure to account for any axial misalignment of the worn turret bore. For an effective repair, the sleeve must be in axial alignment with the original bore and/or the die locating bore in the lower turret. In addition, the interference fitting of the sleeve into the enlarged bore requires unnecessary cost and manufacturing of the sleeve, and an unnecessarily difficult and excessively time consuming installation process. In addition, the requirement of using a simulated mandrel to machine the sleeve is an unnecessarily complicated and time intensive task. The refurbishing process described in this patent increases both cost and machine downtime to effect worn turret bore repairs. Furthermore, future replacement of the sleeves is problematic, if possible at all, due to the interference fit within the bore.
As can be seen from the herein described review of prior attempts to refurbish worn turret bores, there is still a need for an effective, cost efficient method for refurbishing worn turret bores.